Morphological changes of the common bile duct were observed macroscopically and microscopically 20 months after choledochojejunostomy and choledochocolonostomy in rats. The common bile ducts were remarkably dilated in all rats of both experimental models. Choledochal stones consisting of fatty acid calcium and calcium bilirubinate were seen in two of six rats with choledochojejunostomy and two of five rats with choledochocolonostomy. The main pathological change observed in both the groups was remarkable hyperplasia of the choledochal epithelium. Only one rat with choledochojejunostomy revealed normal epithelium with choledochal stone formation. Intestinal metaplasia was seen in two rats with choledochojejunostomy and all with choledochocolonostomy. Squamous pseudostratification of the epithelium indicating atypism was observed in two rats with choledochojejunostomy. Sialomucin producing cells and sulfomucin producing cells were seen in the hyperplastic portion of the epithelium. No malignant alteration of the epithelium was detected. These findings indicate that long-lasting exposure to digestive enzymes and bacteria causes epithelial hyperplasia and further exposure to digestive enzymes plays a major role in appearance of the epithelial atypism. Carcinogenesis of the choledochal epithelium under such an environment will need much more time to be established.